1. Field of the Invention
The subject invention is directed to the field of piezoelectric bender bar acoustic transducers. More particularly, the invention is concerned with the construction of a bender bar from particular piezoelectric elements.
2. Description of the Prior Art
A "bender bar" is a particular type of acoustic transducer which is well known in the art and has been used in underwater sonar applications as well as in borehole monitoring for oil and gas exploration. Today's bender bars are comprised of two stacks of piezoelectric elements which are held together either in a side by side relationship or are attached to opposite sides of an insulative support. The individual piezoelectric ceramic elements in each stack are "poled" so that they either expand or contract upon the application of electric current. The "poling process" is well known in the art and can be defined as the process of exposing a piezoelectric ceramic element to a high voltage current when it is held at its Curie point temperature. The poling process aligns the dipoles of the ceramic in one direction such that the piezoelectric ceramic element will expand if a voltage is applied across the positively poled direction and will contract if voltage is applied across the negatively poled direction.
FIGS. 1 and 2 show the construction of a bender bar 10 and its flexibility under an applied voltage, respectively. FIG. 1 shows the individual piezoelectric ceramic elements 12 are held tightly together between steel end blocks 14 and 16 by stress rods 18 positioned in side rails 20. FIG. 2 shows the positive 22 and negative 24 poles of each piezoelectric ceramic element 12 are electrically connected to a power source 26 or 28. The electrical connection is such that the positive terminal 30 of power source 26 is connected to the positive 22 poled side and the negative terminal 32 is connected to the negative 24 poled side of each piezoelectric ceramic element 12 in the top row. In contrast, the positive terminal 36 and negative terminal 38 of power source 28 are connected to the negative 24 and positive 22 poles, respectively, of each piezoelectric ceramic element 12 in the bottom row. By holding the piezoelectric ceramic elements 12 tightly together with stress rods 18, as shown in FIG. 1, the bilaminar bender bar 10 is caused to flex when each piezoelectric element 12 in the top row expands and each piezoelectric element 12 in the bottom row contracts under an applied voltage, as shown in FIG. 2. The degree and timing of flexing of the bilaminar bender bar 10 can be controlled such that pulsed information can be transmitted to an acoustic media. Likewise, sonic waves which impinge on the bender bar 10 can cause it to flex and the degree of flexure can be translated into a corresponding electrical signal which can be sensed to discern acoustic information.
FIG. 3 shows a trilaminar bender bar 50 which includes several piezoelectric ceramic elements 52 and 53 laminated to opposite sides of a central core 54 of insulative material. Stress rods (not shown) passing through the central core 54 serve to tightly hold the piezoelectric ceramic elements 52 and 53 between steel end blocks 56 and 58. The trilaminar bender bar 50 is secured inside an acoustic instrument such as a hydrophone using the mounting holes 60 in the end block 56. The piezoelectric elements 52 and 53 of the trilaminar bender bar 50 can be electrically connected as described above in conjunction with FIG. 2 such that the bender bar 50 will flex under an applied voltage and act as an acoustic transducer. The poled ends of piezoelectric elements 52 and 53 do not need to be arranged in exactly the same manner as are piezoelectric ceramic elements 12 shown in FIG. 2; rather, all that is required for providing the flexing feature of an acoustic bender bar transducer is that one stack of piezoelectric elements 52 expands under an applied voltage while the other stack of piezoelectric ceramic elements 53 contracts under the applied voltage.
Bilaminar and trilaminar bender bars are expensive to manufacture because the many pieces required to fabricate a bender bar translate into high assembly costs. Two separate stacks of piezoelectric elements must be assembled and then those two stacks must either be cemented together or laminated to opposite sides of a central core. Reducing the number of pieces required for assembling a bender bar would substantially reduce the costs of production. In addition, in both the bilaminar and trilaminar bender bar designs the stress rods must act from a position exterior to the piezoelectric ceramic elements. Such positioning of the stress rods can make control of the mechanical stress placed on the piezoelectric ceramic elements difficult and can reduce the maximum achievable mechanical stress.